Quantification of nitrate leaching from German forest ecosystems by use of a process oriented biogeochemical model Ralf Kiese a, * , Christoph Heinzeller a, b , Christian Werner a, c , Sandra Wochele a , Rüdiger Grote a , Klaus Butterbach-Bahl a a Karlsruhe Institute of Technology, Institute for Meteorology and Climate Research, IMK-IFU Gramisch-Partenkirchen, Germany b Department of Geography, Ludwig-Maximilians University Munich, Germany c LOEWE Biodiversity and Climate Research Centre (BIK-F), Frankfurt, Germany article info Article history: Received 2 May 2011 Accepted 4 May 2011 Keywords: Seepage water Nitrate leaching Forest ecosystems Biogeochemical model Model evaluation Uncertainties abstract Simulations with the process oriented Forest-DNDC model showed reasonable to good agreement with observations of soil water contents of different soil layers, annual amounts of seepage water and approximated rates of nitrate leaching at 79 sites across Germany. Following site evaluation, Forest-DNDC was coupled to a GIS to assess nitrate leaching from German forest ecosystems for the year 2000. At na- tional scale leaching rates varied in a range of 0e>80 kg NO 3 eN ha 1 yr 1 (mean 5.5 kg NO 3 eN ha 1 yr 1 ). A comparison of regional simulations with the results of a nitrate inventory study for Bavaria showed that measured and simulated percentages for different nitrate leaching classes (0e5 kg N ha 1 yr 1 :66% vs. 74%, 5e15 kg N ha 1 yr 1 :20% vs. 20%, >15 kg N ha 1 yr 1 :14% vs. 6%) were in good agreement. Mean nitrate concentrations in seepage water ranged between 0 and 23 mg NO 3 eNl 1 . Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Since several decades, natural ecosystems in Western Europe are exposed to high atmospheric deposition of pollutants such as sulphur and nitrogen. For sulphur deposition major progress has been achieved in the framework of the Gothenburg Protocol: the majority of European countries have reduced their emissions e mainly originating from energy production e by more than 60% between 1990 and 2004, with one quarter of countries having reduced sulphur emission by >80% (Vestreng et al., 2007). However, atmospheric nitrogen deposition has remained at high levels and decreasing trends are not visible yet. In Germany total nitrogen deposition to forest ecosystems are averaging approx. 25 kg N ha 1 yr 1 . In intensively used agricultural areas in the Northwest and South of Germany, annual N deposition may even be higher than 50 kg N ha 1 yr 1 (Gauger et al., 2002). In naturally N limited forest ecosystems, elevated atmospheric deposition of nitrogen can have various adverse environmental effects (Matson et al., 2002), including eutrophication and acidification of terres- trial ecosystems with related impacts on plant and faunal biodiver- sity (Bobbink et al.,1998), eutrophication of surface and groundwater due to nitrate leaching (Dise and Wright, 1995; MacDonald et al., 2002; Gundersen et al., 2006) as well as forcing of global warming due to enhanced emissions of the primarily and secondarily active N-trace gases N 2 O and NO (Butterbach-Bahl et al., 2002; Pilegaard et al., 2006; Venterea et al., 2004), but also increases in C seques- tration due to accelerated forest growth (De Vries et al., 2009). In Germany diffuse water pollution by nitrate leaching is not exclusively limited to intensively used agricultural ecosystems, but has also been reported for forest ecosystems exposed to high loads of atmospheric N deposition. Borken and Matzner (2004) found in their analysis of 57 German forest sites nitrate leaching rates of up to 26 kg N ha 1 yr 1 , with 30% of the sites already showing nitrate leaching rates > 5 kg N ha 1 yr 1 . Having water quality in mind this is of uppermost importance since drinking water in Germany is often gained from “clean” forest watersheds or used for dilution of nitrate polluted water from other areas. Due to the outlined various environmental impacts of chronical N deposition for environmental health, international protocols have been established such as the Gothenburg protocol under the UNECE convention on Long Range Transboundary Air Pollution (LRTAP) aiming at e among others e reducing environmental N threats and, thus N inputs to (semi-) natural ecosystems. Under the LRTAP convention several monitoring programmes have been initiated such as ICP Forests, which stands for the “International Co- operative Programme on Assessment and Monitoring of Air * Corresponding author. E-mail address: ralf.kiese@kit.edu (R. Kiese). Contents lists available at ScienceDirect Environmental Pollution journal homepage: www.elsevier.com/locate/envpol 0269-7491/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2011.05.004 Environmental Pollution 159 (2011) 3204e3214